Do I have a problem with my split vent?

Chromatography Forum: GC Archives: Do I have a problem with my split vent?
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Monday, February 11, 2002 - 03:35 pm:

I have a Varian GC/Ion Trap MS system. The GC is a CP-3800 and the MS is a Saturn 2000. I am new to GC and suspect, but am not sure, that there may be a problem with my split vent. When I am working splitless mode with mobile phase at a constant flow of 1.0 mL/minute, the total flow read from the status screen is 15.3 mL/minute. I thought that when in splitless, all flow went onto the column and thus total flow and column flow reading would be the same. If this is not the case, then where is the other 14.3 mL/minute flow (15.3 mL/minute total minus the 1.0 mL/minute column flow) going? Incidentally, the same happens when working in splitless with constant pressure. I came across this when trying to troubleshoot my instrument after failing to obtain an analyte peak while in splitless mode. Could my failure to get a peak be because my sample is being split even though it is in splitless mode (as a result instrument malfunction) and thus being diluted below the limit of detection. I got a nice separation of the same compound (at higher concentration) when performing split. I would appreciate any insight that you would be willing to share.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Bruce Freeman on Tuesday, February 12, 2002 - 01:12 pm:

I don't know the Varian instrument since I work exclusively with HP/Agilent GC's. HP had a peculiar way of plumbing an instrument (which I happen to like), but I will assime that those peculiarities are irrelevant to this question.

In "splitless" mode ("Grob splitless", to be a little more definitive), the sequence of events is as follows:
1) When preparing to inject, a low flow of carrier gas goes through the inlet liner and onto the column. (There should be no carrier gas going through the inlet liner and out the split vent.) There may be a flow of carrier gas across the septum which never enters the inlet liner (the "septum purge").
2) Injection is made (slow or fast - a matter of opinion or consequence. I like fast, with caveats about acceptable inlet liners). Flow conditions remain as in (1)
3) The sample evaporates relatively slowly (the inlet shouldn't be excessively hot) and carrier gas sweeps the sample out of the inlet liner and onto the column. There should be no split. (At this point, details of solvent, column, "retention gap" and temp. program can matter a lot, but are irrelevant to your observation.)
4) If you left the inlet flow in this state, you'd have terribly tailing peaks, due to exponential dilution of the sample in the inlet. Hence, after a short time (~1/2 minute) you purge the inlet of remaining traces of sample by increasing the flow through the inlet, the excess flow going out the split vent.

In step (4) the additional gas flow might be turned on especially for this function, but I doubt it. More likely (as in the HP GC's) this flow was alway flowing into the inlet, but was exiting the septum purge vent (above the inlet liner). In the HP GC's, this excess septum purge flow is diverted to the split vent by a three-way solenoid valve (trust me -- I don't care to try to explain this) so will be measured as split flow even though there really is no sample splitting!

As to not getting an expected peak, you'd need to provide more details. Splitless injection can be a bit tricky, and you could be making any of a number of mistakes without realizing it. If you want assistance, give us a complete description of your sample (analyte, solvent, concentration) and your instrument (column, detector, temp. program)


Top of pagePrevious messageNext messageBottom of pageLink to this message  By jclark on Wednesday, February 13, 2002 - 05:16 am:

The Varian 1077 (or 1075, I forget which) injector has significant flow at the split vent even when the splitter is off.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Wednesday, February 13, 2002 - 01:38 pm:

Several GCs operate in splitless mode by passing the majority of the flow over the top of the liner, with only a small flow through the column and the majority going out the split vent. On a GC using this system, a low split flow will lead to loss of sample out the least restrictive flow path, i.e. split vent. If you have low sensitivity check septum purge flow if it is adjustable and make sure it is about 1 mL/min. Try increasing flow through the split vent to 4o mL/min. This has two benefits: 1. faster sweep of residual solvent after injection. 2. better response if splitless mode involves diversion of flow over top of liner instead of reducing flow to true splitless levels and stopping split vent flow.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By raj on Thursday, May 20, 2004 - 05:05 pm:

What is the function of the liner?


Posting is currently disabled in this topic. Contact your discussion moderator for more information.